Landracing Forum
Tech Information => Aerodynamics => Topic started by: Jack Gifford on November 20, 2016, 12:59:10 AM
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I'd like to run a screen over the scoop on a blower injector. But I'm guessing that normal screen fabric is quite poor aero-wise, since its made of round-section wire. Anybody know of available screen material that's more aerodynamically optimized? If so, I could use a finer (smaller-aperture) screen that's free-breathing enough, without needing to have a huge screen area.
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Be careful about how much of the open area of the duct is "blocked" by the wires of the screen. You wouldn't think it would be much, but some screen types can reduce the "flow area" by 30/40% Screen mfg's list "high airflow" screen material.
I've had poor inlet screen setups seriously reduce the bhp because of reduced airflow to the engine. I've also had engines overheat after the installation of "rock blocker" screens in from of the heat exchanger(s)/radiator(s).
:cheers:
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Jack, let's back up one step... what is the purpose of this screen? Are you are thinking it is needed to filter air to your very cool engine, then I'm thinking you may need to rethink it.... too many thinks in one sentence :roll:... but if keeping salt out of you motor is the purpose I don't believe you will be able to get screen that fine.
Hopefully the placement of your intake will be in clean air... if it is not... you need to start thinking filter not screen. Our NACA intake has to be where it is so we have to filter or ingest a bunch of salt. The salt we find in our filter is dust.... We use a K&N flat plate filter without oil for our NA motors, and a cone type with our blown motors. Of course you are presented with one of the issues we face, space limitations, that is why we use different filters.
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I expect the scoop to be generally in "clean" air, especially since I may run only pavement LSR. I was planning something like 1/4" screen wire to catch birds, huge bugs, candy bar wrappers, etc. But then I got to wondering about finer screen to catch even smaller stuff. With the small area (maybe 20 sq.in.?) of a forward-facing scoop, I suspect that round-wire window screen would be more restrictive than I'd like. I guess I'll go with the 1/4" screen.
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Jack, I would let the dyno be your friend. Once you've established where you want to be with your engine try a couple of different screens and configurations relating only to filtering and see if you can live with any of them.
Good luck!
Pete
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There's lots of different style KNN filters if you can work one in. Salt coming off the tires during turn out is something that needs to be addressed.
Sid.
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Dont forget filter prescreens as well. They are a very breatheable bag that slides over filter. Can be cut apart and stretched over inlets and such.
AFE makes a "race day" filter, for big chunks. Its basically ta mesh filter without cotton media.
And now power driven diesel (www.powerdrivendiesel.com) has come up with some incredibly high flowing filters that out flow and outfilter any oiled cotton gauze filters on market.
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Jack , you might start by asking how many land speed racers have experienced engine damage from ingested junk .
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Tlhanks for all the comments.
I'm a new-guy at LSR but in other motorsports I've seen a number of instances of "junk" ingested by the open intake of an engine. Admittedly, most have been off-track; but keeping some sort of screen in place would have prevented most damage.
Peter Jack- good idea for dyno testing of screens.
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We used to run a screen between the hat and blower, that was back in the 6-71 days and you could by them off the self from anyone that carried GM parts.
It had fairly fine wire on about 3/16 (6 mesh) spacing. Over time that screen saved engine several times, as we found a variety of things caught by the screen, ie. rocks, nuts, trash, you name it, and yes SSS, the nuts were the metal type.
So; I found that a screen that would stop the big stuff helped, but any finer than what we had would have a very negative effect on air flow.
Rouse
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Johnnie, why is it that you manage to get in a shot or two about Texans no matter what you're saying to us? I assume that when you say that (to me) - you're teasing about restraining that other fine race team from Texas -- Chock Full O' Nuts.
How'd I do?
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SSS,
Just making sure everyone knew I wasn't referring to the organic type, as the engine was behind the driver. :-D
Rouse
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One way of reducing the masking effect of a screen over the inlet is to give the screen a big radius which increases the amount of screen and therefore the number of openings for the air to flow through. You should be able to calculate how much screen you need to provide enough openings to match the scoop area. I would actually add 15 to 20% to make up for the lousy air flow around the screen wire. Once you have the estimated screen area I would suggest a frame made from small tubing or rod that would allow you to tack weld or solder the screen to it and provide support for the screen. As everyone has said, just covering the inlet with flat screen will restrict the air into the blower. You can buy stainless steel screen from McMaster and Carr in almost every opening size you can image.
Rex
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If you google hex airflow straightener or hex airflow straightener design you will see a variety of low restriction honeycomb screens. Some of these are used in airflow sensor boxes. They have some depth and may not be easily fitted to your inlet.
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Aerodynamically you'll find any screen will improve drag, more blockage = less drag. However, Net result will likely be a slower speed as you start starving the engine.
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I bought some screen samples, but haven't really tested them yet. One of them is about 60% open area, with nominal .003" openings, that I'll eventually test on the engine dyno. Anybody have a clue about the relationship between open-area-percentage and actual airflow percentage? I think these samples are simply round-wire screen, but I'm not sure.
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You know the drag formula CdA. Often the Cd is far more important than the "A". Meaning that the shape of the wire can create more drag (or reduce flow) than the area! Round wire having one of the worst Cd's. That is why aircraft have teardrop or oval shaped wires for bracing and struts. Even on a slow 90 mph biplane the bracing wire is oval. Thousand$ spent on CFD probably could get you close but lacking that you will have to do your own dyno or flow tests. You could probably set up a drag-differential test in the garage with just a fan. You couldn't quantify the drag but if you keep the area identical on your test samples you will see their difference in drag. Check out Aircraft Spruce.com for possibilities.
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Add all the screen wire thicknesses together and you'll find you only have half the airflow.
We used a sock made from the same material used in composite vaccuum bag technology
called breather bag on our Mazda Peripheral Port motors and it out "breathed" any commercially
avaible filter. Cheap and excellent. Easy to use and keeps FOD out!.
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I bought some screen samples, but haven't really tested them yet. One of them is about 60% open area, with nominal .003" openings, that I'll eventually test on the engine dyno. Anybody have a clue about the relationship between open-area-percentage and actual airflow percentage? I think these samples are simply round-wire screen, but I'm not sure.
Aero wise, the relationship is quite linear (10% blockage = roughly 10% drag improvement)... its actually more quadratic in nature, but let?s not over complicate things. Engine HP wise I?m not sure, but I?m willing to bet it?s similar.
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Thanks for the comments.
I did not get to do any comparison testing during dyno sessions. Perhaps when I get to a landspeed track I'll be able to do that.
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"Zero power loss using K&N filtration" is a topic discussed on Page 20 in David Vizard's "How to Build Horsepower." He recommends 7 hp per square inch filter area for K&N's. That is the area to cause no power loss.
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Every time I read this post it reminds me of a build for a big off shore boat. It ran 4 BBCs all of 500+ cu. in. and they ran giant 2-15/16 dia inlet tubes on the injectors. The engine builder was a Brit and he "always" ran the neat domed screens over each inlet tube as that was the "proper" way to do it. So one time he had rebuilt one of the engines and was at the dyno testing and tuning and to his surprise he suddenly had an extra 75 horsepower! Same engine just rebuilt but he had forgot to install the inlet screens! He never ran them again.
Rex
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One good reason to mount a big air filter over the inlet end is to provide a reservoir of comparatively calm air for the engine to breathe. This is a big help assure the intake system performs it as designed.